Not applicable.
The present invention relates to the synthesis of adhesive compositions and more particularly to the synthesis of telechelic polymers of selected narrow molecular weight distribution for use in adhesives, coatings, and like applications. For present purposes, xe2x80x9ctelechelicxe2x80x9d polymers are polymers that contain reactive end groups. xe2x80x9cPolytelechelicxe2x80x9d (co)polymers, then, contain two or more reactive pendant groups which often are end groups. For present purposes, xe2x80x9cpolymersxe2x80x9d include homopolymers and copolymers (unless the specific context indicates otherwise), which may be block, random, gradient, star, graft (or xe2x80x9ccombxe2x80x9d), hyperbranched, or dendritic. The xe2x80x9c(co)xe2x80x9d parenthetical prefix in conventional terminology is an alternative, viz., xe2x80x9c(co)polymerxe2x80x9d means a copolymer or polymer, which includes a homopolymer.
Conventional free radical polymerization leads to synthesis of polymers with a fairly broad molecular weight distribution, Mw/Mn (weight molecular weight/number molecular weight), or polydispersity, in the range of 2.5 to 3. Number molecular weight (Mn) relies on the number of molecules in the polymer, while weight molecular weight relies on the weight of the individual molecules. See, e.g., Solomon, The Chemistry of Organic Film Formers, pp. 25, et seq., Robert E. Krieger Publishing Co., Inc., Huntington, N.Y. (1977), the disclosure of which is expressly incorporated herein by reference. The basic theory that applies to the control of the growth of the polymer chains and Mw/Mn ratios in a free-radical initiated polymerization reaction is well documented in the literature by P. J. Flory, JACS, Vol. 96, page 2718 (1952).
State of the art practice used to prepare polymers with a narrow molecular weight distribution in the range of, say, 1.05 to 1.4, rely on living polymerization techniques, such as anionic and cationic polymerization. These ionic living polymerization techniques have several limitations including, for example, restrictions on the types of monomers that can be polymerized, low temperature and purity process requirements, the inability to synthesize high molecular weight polymers, etc. Because of these constraints, ionic polymerization processes are limited to the synthesis of polymers based on styrene, isoprene, isobutylene, and like monomers to produce synthetic elastomers and thermoplastic rubbers.
Telechelic polymers prepared from either living polymers or condensation polymers, such as polyesters, for example, tend to be of low molecular weight, typically on the order of several hundreds to several thousands (e.g., 500-10,000). This low molecular weight limitation makes conventional telechelic polymers impractical for a variety of applications including, for example, adhesives.
Recent work on atom transfer radical polymerization (ATRP) has shown the potential of using this pseudo-living polymerization technique to prepare high molecular weight polymers based on acrylic monomers, vinyl monomers, and other common monomers which polymers exhibit a fairly narrow molecular weight distribution, say, in the range of 1.05 to 1.5. Molecular weights up to 105 have been claimed to have been synthesized by ATRP techniques. See Patten, et al., xe2x80x9cRadical Polymerization Yielding Polymers with Mw/Mn xcx9c1.05 by Homogeneous Atom Transfer Radical Polymerizationxe2x80x9d, Polymer Preprints, pp. 575-576, No. 37 (March 1996); Wang, et al., xe2x80x9cControlled/xe2x80x9dLivingxe2x80x9d Radical Polymerization. Halogen Atom Transfer Radical Polymerization Promoted by a Cu(I)/Cu(II) Redox Processxe2x80x9d, Macromolecules 1995, 28, 7901-7910 (Oct. 15, 1995); and PCT/US96/03302, International Publication No. WO 96/30421, published Oct. 3, 1996, the disclosures of which are expressly incorporated herein by reference.
Disclosed is a method for preparing adhesive polymers which commences with the formation of a poly-telechelic polymer of narrow molecular weight distribution (Mw/Mn), say from about 1-3, by polymerizing one or more radically-polymerizable monomers in the presence of a transition metal, a ligand, and an initiator, under atom or group transfer radical polymerization conditions. In this polymerization step, OH groups are contained on one or more of said initiator, an initiating monomer, a polymerizable monomer, a terminating monomer, or combinations thereof, that is, (i) one or more of the initiator, an initiating monomer, a hydroxy monomer, or combinations thereof; on (ii) one or more of a hydroxy monomer, a terminating monomer, or combinations thereof; or on (iii) one or more of the initiator, an initiating monomer, or terminating monomer, or combinations thereof. The poly-telechelic polymer, then, is chain extended with a chain extension agent, such as a polyisocyanate, to form the adhesive polymer.
Regression analysis reveals that the adhesive properties of the chain extended polymers is dependent primarily upon the Mn of the telechelic polymer and the hydroxyl monomer and/or initiator used in forming the telechelic polymers. Data demonstrating such adhesive properties is set forth herein.
In the polytelechelic polymer formation step of the process, atom or group transfer radical polymerization conditions are used. Such conditions can be found described in, for example, the art cited above and incorporated herein be reference. Included in this step are a transition metal, a ligand, and an initiator.
Preferred transition metals are Cu+1, and Co+1, although many other transition metals have been disclosed in the art and may find advantage in the present invention. Cu+1 halides, for example, arc described with respect to catalyzed reactions of organic polyhalides with vinyl unsaturated compounds are well known by Bellus, Pure and Applied Chemistry, Vol. 57, No. 12, pp. 1827-1838 (1985). Complexing of transition metal halides with organic ligands as part of the initiator system is described in U.S. Pat. No. 4,446,246, for example. Cu+1 halide-bipyridine complexes with active organic halide compounds are described to react with vinyl unsaturated compounds by Udding, et al., J. Organic Chemistry, Vol. 59, pp. 1993-2003 (1994). Organocobalt porphyrin complexes (alkyl cobaloximes) are described in the polymerization of acrylates by Wayland, et al., JACS, Vol. 116, pp. 7943-7966 (1994). Cu+1 carboxylate complexes formed from thiophene carboxylates are described by Weij, et al., Polymer Preprints, Vol. 38, No. 1, pp. 685-686 (April 1997). The disclosures of the foregoing references are expressly incorporated herein by reference.
The generation of radical intermediates by reacting some transition metal species, including salts and/or complexes of Cu, Ru, Fe, Va, Nb, and others, with alkyl halides, R-X, is well documented (see Bellus, Pure and Appl. Chem., 1985, 57, 1827; Nagashima, et al., J. Org. Chem., 1993, 58, 464; Seijas, et al., Tetrahedron, 1992, 48(9), 1637; Nagashima, et al., J. Org. Chem., 1992, 57, 1682; Hayes, J. Am. Chem. Soc., 1988, 110, 5533; Hirao, et al., Syn. Lett., 1990, 217; Hirao, et al., J. Synth. Org. Chem., (Japan), 1994, 52(3), 197; Iqbal, et al., Chem. Rev., 94, 519 (1994); Kochi, Organometallic Mechanisms and Catalysis, Academic Press, New York, 1978. Moreover, it also is known that R-X/transition metal species-based redox initiators, such as Mo(CO)6/CHCl3, Cr(CO)6/CCL4, Co4(CO)12/CCl4, and Ni[P(OPh))3]4/CCl4, promote radical polymerization (see Bamford, Comprehensive Polymer Science, Allen, et al., editors, Pergamon: Oxford, 1991, vol. 3, p. 123). The participation of free radicals in these redox initiator-promoted polymerizations was supported by end-group analysis and direct observation of radicals by ESR spectroscopy (see Bamford, Proc. Roy. Soc., 1972, A, 326, 431). The disclosures of the foregoing references are expressly incorporated herein by reference.
Ligands useful in the polytelechelic polymer formation step of the process also have been disclosed in the literature, such as set forth above. Such ligands most readily are halides; although, bipyridyls, mercaptides, triflates (CuOSO2CF3, J. Am. Chem. Soc., 95, 1889 (1973), incorporated herein by reference), olefin and hydroxyl complexes (see, Cotton and Wilkinson, Advanced Inorganic Chemistry, 3rd Ed. Chapter 23, John Wile and Sons, New York, N.Y. (1972; xe2x80x9cInorganic and Organometallic Photochemistryxe2x80x9d, M. S. Wrighton, Editor, ACS-Advances in Chemistry Series, 168 (1978); and Srinivasan, J. Am. Chem. Soc., 85, 3048 (1963), incorporated herein by reference) can be used as necessary, desirable, or convenient. The disclosures of the foregoing references are expressly incorporated herein by reference.
Initiators also have been disclosed in the literature. Representative of such initiators include, for example, 2-hydroxyethyl 2-bromopropionate, 2-hydroxyethyl 4-bromopropionate, methyl 2-bromopropionate, 1-phenyl ethyl chloride, 1-phenylethyl bromide, chloroform, carbon tetrachloride, 2-chloropropionitrile, lower alkyl (C1-C6) esters of 2-halo-lower alkyl carboxylic acids (e.g., ethyl 2-bromoisobutyrate), xcex1, xcex1xe2x80x2-dichloroxylene, xcex1, xcex1xe2x80x2-dibromoxylene, hexakis(xcex1-bromomethyl)benzene, and like. Obviously, halide initiators have been taught by the art to be preferred and such initiators serve quite efficaciously in the present invention. It should be observed, further, that photoinitiators also can be used, such as taught by M. P. Greuel, xe2x80x9cLiving Free-Radical Polymerization Using Alkyl Cobaloximes as Photoinitiatorsxe2x80x9d, Doctoral Thesis, University of Akron, December 1992. The disclosures of the foregoing references are expressly incorporated herein by reference.
Referring now to radically-polymerizable monomers, broadly, such monomers include any ethylenically unsaturated monomer or oligomer which can be (co)polymerized in the presence of a the initiator. In adhesives technology, acrylic or acrylate compounds find wide acceptance in industry. Another suitable class of ethylenically unsaturated compounds are vinyl compounds, while a third broad class are compounds containing backbone ethylenic unsaturation as typified by ethylenically unsaturated polyester oligomers. For terminating or capping the polymer ends with OH functionality, monomers modified to contain such functionality are used in the polymerization step of the present invention.
Referring with more particularity to reactive acrylic or acrylate monomers or oligomers, a variety of monoacrylate monomers find use in accordance with the present invention. Monoacrylates include, for example, allyl (meth)acrylate, C1-C22 alkyl and cycloalkyl (meth)acrylates, such as, for example, butyl acrylate, 2-ethylhexyl acrylate, isooctylacrylate, amyl acrylate, lauryl acrylate, iso-propyl acrylate, and the like, and corresponding monomethacrylates which include, for example, benzyl methacrylate, stearyl methacrylate, decyl methacrylate, cyclohexyl methacrylate, and the like, and mixtures thereof. The foregoing monomers are merely representative and not limitative of the list of acrylate and methacrylate monomers suitable for use in the present invention as those skilled in the art will appreciate.
Other suitable reactive compounds for use in the present invention include, for example, acrylated epoxy resins, acrylated silicone resins, acrylated polyurethane resins, and the like and mixtures thereof. Such acrylate-functional compounds are well known in the art and little more about them need be stated here.
Hydroxyl-containing acrylic monomers include hydroxyl derivatives of those monomers named above (e.g., hydroxy ethyl acrylate or hydroxy ethyl methacrylate), and the like, and mixtures thereof.
Hydroxy-functional initiators can be used in order to cap one end of the polymer (i.e., initiate the polymer). Alternatively, a pre-monomer can be used to start the polymerization which then proceeds with non-functional monomers. The other end of the polymer can be terminated with such functionality by choice of monomer which can be functional or a functional monomer (for example, allyl alcohol) can be post-polymerization added to cap the polymer with desired hydroxyl functionality. In this regard, it will be appreciated that the efficiency of hydroxyl incorporation into the telechelic (co)polymers is much greater when a hydroxy initiator or hydroxy initial monomer is used, rather than end-capping with a functional monomer, as those skilled in the art will appreciate. Mono and di-hydroxyl functional telechelic (co)polymers are preferred for use in the present invention; although, high functionality may be useful on occasion as is necessary, desirable, or convenient.
Polyisocyanates, preferably diisocyanates, are conventional in nature and include, for example, hexamethylene diisocyanate, toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), m- and p-phenylene diisocyanates, bitolylene diisocyanate, cyclohexane diisocyanate (CHDI), bis-(isocyanatomethyl) cyclohexane (H6XDI), dicyclohexylmethane diisocyanate (H12MDI), dimer acid diisocyanate (DDI), trimethyl hexamethylene diisocyanate, lysine diisocyanate and its methyl ester, isophorone diisocyanate, methyl cyclohexane diisocyanate, 1,5-napthalene diisocyanate, xylylene and xylene diisocyanate and methyl derivatives thereof, polymethylene polyphenyl isocyanates, chlorophenylene-2,4-diisocyanate, and the like and mixtures thereof. Triisocyanates and high-functional isocyanates also are well known and can be used to advantage; although, diisocyanates are presently preferred. Aromatic and aliphatic diisocyanates, for example, (including biuret and isocyanurate derivatives) often are available as pre-formed commercial packages and can be used to advantage in the present invention. As with conventional urethane reactions, there should be a slight to moderate excess of isocyanate equivalents compared to the hydroxyl equivalents of the telechelic (co)polymers being chain extended.
The chain extended poly-telechelic polymers find wide use in formulating adhesives. Such adhesive polymers retain bond strength by dint of their higher molecular weight, but also exhibit good peel properties as do lower molecular weight polymers while still maintaining desired viscosities of prior art adhesives. Thus, the chain extended adhesive polymers exhibit a combination of bond strength which is expected of high molecular weight polymers, while also exhibiting peel properties expected of much lower molecular weight polymers. Such peel properties and good viscosities are believed to result because of the narrow molecular weight distribution of the poly-telechelic polymer intermediates synthesized in accordance with the precepts of the present invention.